This invention relates to glasses having a high refractive index and a low specific gravity for use as spectacle lenses.
In order to replace the hitherto lightest-weight and highest refractive index glasses (nd=1.70, s=3.0) used as spectacle lenses by an even higher refractive index glass (e.g., nd&gt;1.79), so as to produce spectacle lenses having higher dioptric values with even smaller edge thicknesses (for negative lenses) and/or even smaller center thicknesses (for positive lenses) than previously done, thereby enabling further enlargement of the optically effective field of vision while also improving cosmetic appearances, the specific gravity must be approximately 3.3 if the weight of the glass lens is to remain approximately the same. In a glass with a refractive index of nd=1.79 and a specific gravity of approximately 3.7, the eventual glass weight increase is approximately 12 percent for negative and positive lenses as compared to a glass having a refractive index of nd=1.70 and a specific gravity of 3.0.
Optical glasses with refractive indexes nd of 1.76-1.80 and specific gravities s of .ltoreq.4.5 are known, but as a result of their weight are not suited for use as glasses for spectacle lenses, even though such glasses can be produced by continuous mass-production means.
Examples of these unsuitable glasses are heavy flint glasses as follows:
______________________________________ SF 6 nd = 1.8051 vd = 25.43 s = 5.18 SF 11 nd = 1.7847 vd = 25.76 s = 4.74 SF 14 nd = 1.7612 vd = 26.53 s = 4.54 ______________________________________
The known LaF glasses with a refractive index nd of &gt;1.77 but a lower dispersion than heavy flint glasses have specific gravities of &gt;4.0. Examples of the known LaF and LaSF glasses are as follows:
______________________________________ LaF 25 nd = 1.7843 vd = 41.30 s = 4.45 LaF 22 nd = 1.7818 vd = 37.1 s = 4.21 Laf 9 nd = 1.7950 vd = 28.39 s = 4.96 LaSF N 3 nd = 1.8080 vd = 40.75 s = 4.68 LaSF 8 nd = 1.80741 vd = 31.61 s = 4.87 ______________________________________
Further high refractive index optical glasses, e.g., as disclosed by German Patent Specification No. 12 60 712, having refractive indexes nd of 1.75-1.80 and specific gravities .ltoreq.4.0 are known, but they are not suitable for continuous production of solid glasses (e.g., glasses for spectacle lenses) and crucible melting due to their pronounced tendency towards crystallization. This crystallization tendency is determined by the magnitude of the crystal growth speed and the crystallization temperature range in relation to viscosity.
Glasses having a refractive index &gt;1.75 consist of 14-46 percent by weight of alkaline earth metal oxides, including a heavy BaO content .ltoreq.10 percent by weight. The ZnO content is 10-20 percent by weight and the TiO.sub.2 -content is 10-25 percent by weight. Such glasses contain 12-20 percent by weight of SiO.sub.2 and 8-20 percent by weight of B.sub.2 O.sub.3 as glass components, the sum of glass forming components being &lt;31 percent by weight. The color of such high titanium-bearing glasses must be influenced by rapid cooling in such a manner that the glass has a strongly marked titanium color (yellow-brown). The glasses must be cast at a viscosity &lt;100 poises because their strong tendency towards crystallization precludes tank production and automatic processing at higher viscosities, e.g. between 200 to 600 poises. The ZnO content of 10-20 percent by weight in conjunction with the high TiO.sub.2 -content of 10-25 percent by weight is unfavorable for these glasses.